1. Technical Field
The embodiments herein generally relate to metallic nano-products such as metallic nano-antennas, metallic nano-rods, metallic nano-wires, metallic nano-prolate spheroids and metallic nano-oblate spheroids used in bio medical applications like sensing, imaging and photo thermal therapy. The embodiments herein more particularly relates to a method of producing the metallic nano-products using fatty acids and without using a shape regulating surfactants.
2. Description of the Related Art
Up-conversion of infrared (IR) light has been realized in a number of ways ranging from multi-photon processes, pair energy transfer and through phase matched nonlinear processes in rare earth and phosphor materials. All of these approaches have been improved over several years and in particular in fiber-based geometries where long interaction lengths and high intensities resulting from mode confinement provide obvious advantages. An obstacle to an efficient up-conversion of infrared light is the overall interaction cross section for the up-conversion process. Regardless of the specific mechanism, two or more particles, photons with photons, or photons with phonons must be combined to generate a visible photon in emission. At its basic level, a sensitization problem exists, which requires the cross sections or likelihood of absorption and emission to be greatly enhanced. This is of particular importance in a light bulb or other standard illumination source, when low power fluxes and thin interaction lengths are an additional constraint.
A molecule which absorbs energy near the particle surface at which the plasmon resonance of a nanoscale metallic particle occurs, experiences the enhanced field and absorb energy at a higher rate. Similarly, a radiating molecule can emit energy at the particle surface where plasmon resonance occur faster than it could into free space. The enhanced absorption behavior has been measured using dyes relevant to dye sensitized solar cells. The nanoscale metallic structures can act as high-gain antennae for light sensitive molecules similar to a metal rod acting as a gain antenna for a television set. When this nanoscale structure is much smaller than the wavelength of light, the structure concentrates, absorbs and transfers energy. For example, an antenna used to collect approximately one meter wavelength radio waves is sized to a similar one meter length to detect the radio waves in the air.
Gold nano-particles in a shape of a rod such as gold nanorods with uniform configuration have a strong absorption band in a region extending from visible light to near infrared rays and it is possible to change its absorption peak positions easily by controlling the configuration thereof. Gold nanorods have high aptitude as near-infrared probes because the modification of their surface enables change in their physical properties. The tuneable NIR absorbance of gold in conjunction with its low cytotoxicity has fueled research in the synthesis of rod-like gold nanocrystals for a wide range of biomedical applications such as sensing, imaging and photo-thermal therapy.
There are various conventionally known methods of manufacturing gold nanorods. They are electrolytic method, chemical reduction method and photo-reduction method.
In the electrolytic method, a solution containing a cationic surfactant is electrolyzed by a constant current and gold clusters are leached from a gold plate at the anode thereby generating gold nanorods. The surfactant used is a quaternary ammonium salt having a structure with four hydrophobic substituents bonded to a nitrogen atom. In addition tetradodecylammonium bromide (TDAB) is added to avoid formation of autonomous molecular assembly. Here the source of the gold supply is the gold clusters that are leached from a gold plate at the anode. In this method, the gold salt such as chlorauric acid is not used. Ultrasonic waves are radiated during electrolysis and a silver plate is immersed in the solution to accelerate the growth of the gold nanorods. The electrolytic method is characterized by the fact that the change of the area of the silver plate to be immersed separately from an electrode enables to control the length of the rod to be generated. The adjustment of the rod length enables setting of the absorption band in the near-infrared region from the vicinity of 700 nm to the vicinity of 1200 nm. If the reaction condition is uniformly maintained, gold nanorods with a uniform configuration can be manufactured to an extent. However, since the surfactant solution used for the electrolysis is a complex system containing excessive quaternary ammonium salt, cyclohexane and acetone and indefinite elements such as ultrasound wave radiation, it is difficult to theoretically analyze a cause-effect relationship between the configuration of the gold nanorods to be generated and various manufacturing conditions and to optimize the manufacturing conditions for the gold nanorods. Furthermore, it is not easy to scale up the manufacture of gold nano rods in electrolytic process due to the nature of the electrolysis thereby making it unsuitable for the large-scale manufacture of gold nanorods.
In the chemical reduction method, sodium borohydride (NaBH4) reduces chlorauric acid to generate gold nano-particles. These particles are considered as “seed particles” and growing them in a solution results in the production of the gold nanorods. The length of the gold nanorods to be generated is determined according to the quantitative ratio of the “seed particles” to the amount of chlorauric acid added to the growth solution. The chemical reduction method helps to generate longer gold nanorods in comparison with the electrolytic method. A gold nanorod produced by using chemical reduction method has an absorption peak of over 1200 nm in the near-infrared region. But the disadvantage of chemical reduction method is that it requires two reaction baths. One for preparation of seed particles and another for reaction to grow the “seed particles”. Furthermore, it is difficult to increase the concentration of the gold nanorods generated and the generation concentration of the gold nanorods is very less as compared to the electrolytic method.
In the photo-reduction method, the chlorauric acid is added to substantially the same solution as that in the electrolytic method and exposed to ultraviolet irradiations. The ultraviolet irradiation results in the reduction of the chlorauric acid. A low-pressure mercury lamp is used for ultraviolet irradiation. In the photo-reduction method, gold nanorods can be directly generated without producing seed particles. The length of the gold nanorods can be controlled by the irradiation time. By this method, an excellent uniform configuration of the gold nanorods can be generated. As the large quantity of spherical particles coexist after reaction, it is necessary to separate the spherical particles by centrifuging. However, by this method, the ratio of the spherical particles is small so the separation is unnecessary. Furthermore, the reproducibility is excellent. The gold nanorods obtained after using a standard operation are of same size. But, the photo-reduction method is time consuming as it requires 10 hours or more for the completion of a reaction. The particles obtained do not have an absorption peak over 800 nm. Also, the light used from the low-pressure mercury lamp is harmful to the human body.
Moreover, a fundamental problem with all of these technologies is the need for surfactants, such as cetyltrimethylammonium bromide (CTAB), in order to induce the anisotropic particle growth in aqueous solution. Also, these surfactants are cytotoxic in nature.
Hence, there is a need to provide an alternate synthetic method which alleviates the need for shape-regulating surfactants.
The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.